In recent years, for purposes such as power conversion and power control, or high-power amplification/oscillation and the like, semiconductor elements called power devices have been widely used. These semiconductor devices or elements include bipolar transistors, IGBT (Insulated Gate Bipolar Transistors), power ICs, and MOSFETs and the like, and they all have the merit that their conversion or control efficiency is high and they are small.
However, in these semiconductor elements, because a large current flows, the amount of heat produced is large, there is a risk of destruction or changes in characteristics occurring due to heat produced by the elements themselves, and so it is necessary them to be cooled efficiently. To this end, various cooling structures have been proposed up to now, and as one example of these, a conventional semiconductor device having a cooling structure is shown in FIG. 2 hereof.
Referring to FIG. 2, a semiconductor device 100 is made by providing an insulating plate 103 and a metal plate 104 between a semiconductor element 101 and a heat sink 102. The semiconductor element 101 is a power device such as a power transistor, a power FET, or an IGBT, and is made of a silicon semiconductor or the like. When the semiconductor element 101 operates, due to joule heat, heat is produced and its temperature rises. When the temperature of the semiconductor element 101 rises, its on-resistance increases, and when feedback control or the like is being carried out, further voltage is applied in order to make current flow, current flows, and consequently the joule heat increases and the temperature rises further. Because of this, breakage of the semiconductor element 101 occurs. To prevent this, the semiconductor device 100 is provided with a heat sink 102.
The heat sink 102 is made of copper or aluminum or the like, which have good thermal conductivity, and radiates heat produced by the semiconductor element 101. Consequently, it can prevent increase of the temperature of the semiconductor element 101. Accordingly, it is shaped so as to have a large radiating area.
The insulating plate 103 electrically insulates the semiconductor element 101 and the heat sink 102 from each other. For this, a material which has good thermal conductivity but is electrically insulating, such as SiN, is used.
The metal plate 104 is for supporting the semiconductor element 101 and the insulating plate 103, and is also for conducting heat from the semi-conductor element 101 to the heat sink 102. For this, a copper plate or the like which has good thermal conductivity and is a firm material is used.
The heat sink 102 is for diffusing heat produced in the semiconductor element 101 and conducted to it through the insulating plate 103 and the metal plate 104; it has a structure that makes its area of contact with air large, and is made of copper, aluminum, or molybdenum or an alloy of these. The semi-conductor element 101, the insulating plate 103, the metal plate 104 and the heat sink 102 are bonded together with solder or brazing filler material or the like.
In the conventional semiconductor device described above, there was the shortcoming that because compared to the thermal expansion coefficient of the semiconductor element the thermal expansion coefficient of the copper or aluminum or other metal is large, the metal expanding along with the production of heat by the semiconductor element causes thermal stress to arise, the device itself bends, and detachment and/or cracking or the like occur. To solve this kind of shortcoming, technology whereby the bonding material between the insulating plate and the heat sink is contrived to bond firmly has been disclosed, for example in JP-A-2002-43482. A hard brazing filler material including an active element is used for this bonding material.
As described above, in the conventional semiconductor device shown in FIG. 2, there was the shortcoming that because there is a difference in thermal expansion coefficient between the metal and the other materials, warping occurs in the device when the semiconductor element produces heat, and as a result detachment and cracks and so on occur. Thus there have been the problems that it has not been possible to apply a high load to the semiconductor element, its full potential performance cannot be exploited, a forced cooling device such as a water-cooling device or a fan or the like has been necessary, and the equipment has become large.
When the technology disclosed in JP-A-2002-43482 is used, because the thermal conductivity of the bonding material itself is poor, the performance of the semiconductor device in dispersing heat by conducting it to the heat sink is lost, and because as a result the semiconductor element is not efficiently cooled, it has not been possible to exploit the full potential performance of the semiconductor element. Also, the bonding material is expensive and has been a cause of increased cost of the semiconductor device.
Accordingly, technology has been demanded for reducing warping of a semiconductor device occurring when a semiconductor element produces heat has been awaited.